The recovery of heavy oils using horizontal wells has been carried out on an increasing scale in Canada and other countries in recent years. Typical practice in fields such as those in Winter, Senlac, Long Lake, Cactus Lake and Haytar, all in Canada, involves the construction of horizontal production wells about 1000 m in length at the top of the reservoir. Oil is produced by pumping and the recovery is limited eventually by the watering out of the production. Water displaces and replaces the oil as it rises from aquifers lower in the reservoir. The advantage of horizontal wells over conventional, near-vertical ones is that the volume of the reservoir affected by the rising water is much greater and oil is drawn by each well from a larger area. In favourable cases, cumulative production quantities of 100,000 barrels or more of oil are achieved at economic production rates. These are sufficient to more than pay for the cost of the horizontal well and its operation. The disadvantage of the process is that a recovery of only about 5% of the oil in place is achieved. Much of the original oil is left behind in the unswept and water-swept regions.
The inventor has previously proposed an approach for the recovery of heavy oils that involves the use of vaporized solvents such as ethane, propane, or butane. In this method use is made of gravity to cause the oil diluted by the solvent to fall to the base of the reservoir with its initial pore volume becoming filled by solvent vapor. The process is effective if appropriate reservoir conditions can be achieved. In particular, for economic application of the process, it is necessary to have a large mass transfer area available since diffusive mixing is slow. The pressure needs to be close to the vapor pressure of the injected solvent since light hydrocarbon vapors only have a high solubility in the oil when they are close to their dew point. This limited the most economic applications of the process to reservoirs where the pressure can be controlled appropriately, i.e. to about 500 psig for ethane, 110 psig for propane and 20 psig for butane.
Propane has a particular advantage as a solvent for use in such recovery techniques because it allows the precipitation of considerable asphaltene material in the reservoir thus producing an upgraded, less viscous, and more valuable oil product. For propane to be used by itself at normal reservoir temperatures a pressure of the order of 110 psig is required.
In a previous application for patent, the inventor proposed a process that uses saturated hydrocarbon solvent vapor (typically ethane or propane) in conjunction with horizontal wells to mobilize and recover viscous oils and bitumens from reservoir of hydrocarbons in which a bottom water zone (aquifer) is used to deliver the solvent vapor to the base of the reservoir. A hydrocarbon solvent in the vapor phase is preferably injected at reservoir temperature into an aquifer underlying the deposit. The hydrocarbon solvent vapor is essentially insoluble in water, while strongly soluble in oil, with the consequence that there are no heat or material losses to the water layer. Furthermore, the water in the bottom water zone will be mobilized and underride the lighter diluted oil and assist in moving it towards the production well. Use of this process produces a much larger interfacial area for mass transfer. However, the pressure requirement probably limits the general applicability of using propane alone in such a method to relatively shallow bitumen reservoirs.
In this patent document, a further improvement of the above described hydrocarbon recovery process includes the injection of a non-condensible displacement gas into a reservoir with a hydrocarbon solvent. The displacement gas should be at a pressure sufficient to limit the rate of ingress of water into the recovery zone to a small manageable value. The process is operated so that some of the injected displacement gas is produced from the horizontal production wells located at the base of the reservoir. The injection may be into the bottom water layer if this exists or, if it is practical (as it usually will be with conventional heavy oils but not with bitumen), into horizontal injection wells located at the top of the reservoir, parallel to and probably displaced horizontally from the producers. Gas is injected into the reservoir from these wells and the production is controlled so that there is a net but manageable small flow of gas from injector to producer. In conventional reservoirs such an operation may produce a small recovery of heavy oil by itself, but this will be limited because of the tendency of the low viscosity gas to flow through the oil as fingers and bypass it. The hydrocarbon solvent should be a liquefied but vaporizable solvent such as butane or propane. Confined evaporation of this liquid solvent in the displacement gas maintains a concentration of solvent vapor in the gas phase. The rate of injection of liquefied solvent is preferably controlled so that essentially all of the solvent is vaporized before the gas stream reaches the production well. This vaporized solvent diffuses within the gas fingers and dissolves into the heavy oil reducing its viscosity. The diluted oil becomes mobile and accumulates at the bottom of the gas fingers so as to tend to seal them off. However the steady flow of gas results in displacement and diluted oil is produced. The rate of injection of liquefied solvent is controlled so as to maintain a practical concentration in the produced oil (2-50% by weight).
A limitation of this approach can be that the fingers of gas produced by the initial injection sweep only a small volume of the reservoir and, as a result, there is only a limited interfacial area for the solution of the vaporized solvent into the bitumen. Mixing is limited and, as a result, the production rate may be limited. Therefore, in a further aspect of the invention, use is made of the flow channels that have been developed in a reservoir that has been previously produced by conventional water displacement of the type described above. In the water-flooded reservoir there are numerous passages in which water has been flowing through the oil sand. In this further aspect of the invention, the displacement gas is injected with liquefied but vaporizable solvent into the horizontal well(s) already located at the top of the reservoir and to displace the water downwards to new horizontal production wells drilled at the base of the reservoir, parallel to the injectors. The producers may be located directly below each injector or in between. Non-condensible gas is allowed to flow through these interconnected passages and to displace water to the production well. Again, as in the previous processes, the pressures are arranged so that there may be a small inflow of fluids from outside the recovery region but only at a controllable rate. The liquefied but vaporizable solvent such as propane or butane is injected at a rate controlled to provide a practical concentration of solvent in the produced fluids (2-50% by weight).
An important advantage of the process is that the tendency for the vaporized solvent to rise will cause the swept region to increase in size upwards.
The solvent is recovered from the produced oil and recycled to the injectors. It is believed that significant increase in recovery from horizontal well projects in areas such as Lloydminster, Alberta, Canada, can be achieved by use of this invention.
There is thus provided in accordance with one aspect of the invention, a method for the recovery of hydrocarbons from an underground reservoir of hydrocarbons, the method comprising the steps of:
injecting a displacement gas and a liquid vaporizable hydrocarbon solvent into an injection well lying in the underground reservoir to mobilize hydrocarbons in the underground reservoir; and PA1 producing mobilized hydrocarbons together with displacement gas and liquid vaporizable hydrocarbon solvent from a production well lying in the underground reservoir spaced from the injection well. PA1 a source of liquid vaporizable hydrocarbon solvent; PA1 a source of displacement gas; PA1 a first injection well drilled horizontally into the reservoir, the injection well having a portion open to fluid communication with the reservoir and being connected to the source of liquid vaporizable hydrocarbon solvent; and PA1 a first production well drilled horizontally into the reservoir, and spaced from the injection well, the first production well including a pump for pumping oil from the well. PA1 drilling a second horizontal well into the reservoir of hydrocarbons spaced from the first horizontal well; PA1 establishing a communication path with flow of displacement gas from the first horizontal well to the second horizontal well; PA1 injecting a hydrocarbon solvent into the reservoir of hydrocarbons through the first horizontal well while injecting displacement gas such that hydrocarbon solvent in the vapor state is present in the displacement gas in the reservoir; and PA1 producing hydrocarbons and displacement gas from the second horizontal well.
In another aspect of the invention, the displacement gas is injected into the injection well until a communication path is established between the injection well and the production well and then the liquid vaporizable hydrocarbon solvent is injected along with the displacement gas.
Preferably, the mobilized hydrocarbons are produced along a predominantly horizontal production well in the underground reservoir.
In another aspect of the invention, where the reservoir has previously been produced by an existing horizontal production well lying at the top of the underground reservoir, the displacement gas is injected into the existing production wells, and in addition, production wells are created by drilling a new production well into the underground reservoir below the existing production well.
Methane and nitrogen are preferred for the displacement gas while butane and propane are preferred for the liquid vaporizable hydrocarbon solvent.
In a further aspect of the invention, there is provided apparatus for the recovery of hydrocarbons from a reservoir of hydrocarbons, the apparatus comprising:
A solvent stripper is preferably connected between the first injection well and the first production well.
In another aspect of the invention, a method is provided for the production of hydrocarbons from a reservoir of hydrocarbons having a first horizontal well drilled into the reservoir of hydrocarbons, the method comprising the steps of:
The hydrocarbon solvent is preferably injected in the liquid phase and vapor from the hydrocarbon solvent preferably saturates the displacement gas. The hydrocarbon solvent is preferably selected from the group consisting of propane and butane.
Further aspects and advantages of the invention are described in the detailed description that follows and set out in the claims that follow.